1. Field of the Invention
The present invention relates to an electrophotographic recording system using an electrophotographically sensitive plate, and more particularly, to an electrophotographic recording system capable of erasing the residual charge on the electrophotographically sensitive plate by the use of an ionic erasing method.
2. Description of the Prior Art
Marked development has been achieved of late in the art of electrophotographic recording systems by virtue of various improvements proposed in electrophotography.
One well known electrophotographic recording system uses an electrophotographically sensitive plate consisting at least of three layers; a front insulating layer, a photoconductive sensitive layer, and a rear electroconductive layer, wherein a charge image of specific polarity is formed on the front insulating layer, the charge image is transferred to a recording paper by means of a transferring roller, and the residual charge on the front insulating layer is erased by a suitable method such as cleaning, charge-erasing, or corona charging at reverse polarity.
This conventional recording system will be described in detail by referring to FIGS. 1 and 2.
FIGS. 1a through 1d schematically illustrate steps of the electrophotographic process. In FIG. 1a, the reference numeral 4 denotes an electrophotographically sensitive plate (hereinafter referred to as a photosensitive plate) constituted integrally of a front insulating layer 1 such as a Mylar film, a photoconductive layer 2 comprised of CdS, Se-Te or the like with a lowered impedance when exposed to light and a rear surface electrode 3.
The step of forming a charge image on the photosensitive plate 4 by a negative corona charging device 5 will be described. For explanatory simplicity, assume that the photosensitive plate 4 is exposed not in the left half but in the right as shown in FIG. 1a.
The impedance of the photoconductive sensitive layer 2 is lowered in the exposed part with the result that the quantity of charge imparted from the coronoa charging device 5 to the front insulating layer 1 increases to cause the surface of the insulating layer to be strongly charged at a negative polarity. In the unexposed part, the impedance of the photoconductive sensitive layer 2 is high. Accordingly, the front insulating layer 1 is weakly charged as shown in FIG. 1b.
With reference to FIG. 2a, the solid line indicates the relationship between the potential at the front insulating layer and the illuminance in the photosensitive plate 4 with CdS layer 2. In FIG. 2a, the abscissa represents the dose of light (lux-sec), that is, the product of the illuminance (lux) and the time of exposure (sec). The ordinate represents the potential V.sub.1 (volts) at the front insulating layer 1. FIG. 2a indicates that the potential approaches a constant value Ve as the dose of light increases, approaches a value Vd as the dose of light decreases, and assumes a value according to illuminance in the intermediate range of light dose.
The dependence of the dose of light upon the potential is heavily dependent on the material of the photoconductive sensitive layer 2, as well as on the method of making it, and is not inherent in CdS. Assume that the photosensitive plate 4 is constituted of a front insulating layer 1 of Mylar 15 microns thick, and a photoconductive sensitive layer 2100 microns thick formed of CdS powder containing an impurity (0.005 wt.% copper) with a binder (10 wt.% acryl resin). Experimentally, this photosensitive plate was charged by use of a charger 5 in such manner that the charger was moved above the photosensitive plate at a speed of 300 mm/sec as a corona charge of -6.0 kV with an effective width of 30 mm was being applied by a tungsten wire of 50 microns in diameter with the result that the potentials Ve and Vd were -1300 V and -600 V, respectively.
Then, as shown in FIG. 1c, an electrostatic recording paper 8 comprising a low resistance paper 7 on which a high resistance layer 6 is attached is brought into contact with the front insulating layer 1 of the photosensitive plate 4 by a transferring roller 9 connected to a voltage source 10 offering a transferring voltage Vb.sub.1 whereby the charge on the front insulating layer 1 is transferred to the surface of high resistance layer 6 of the electrostatic recording paper.
Assume that the transferring roller 9 is grounded directly. Then, the transferring is effected only when the potential at the front insulating layer 1 exceeds the transferring initiation voltage Vo.sub.1 which is developed due to the discharge or an electric field across a narrow gap between the photosensitive plate 4 and the electrostatic recording paper 8 because the charge on the front insulating layer 1 is transferred to the recording paper across this gap.
The transferring initiation voltage Vo.sub.1 depends upon the condition of the surface of the recording paper 8 and of the front insulating layer 1 and usually ranges from .+-.300 to .+-.500 V. After the charge is transferred, a potential of about Vo.sub.1 + .alpha. (V.sub.1 - Vo.sub.1) is considered to remain on the front insulating layer. (Note: The symbol .alpha. stands for a value which depends upon the capacitance of the front insulating layer 1 and the high resistance layer 6; it is normally 0.2 to 0.3) For example, assume .alpha. is 0.2. Then, the potential which remains on the layer 1 (or the residual potential on the layer 1) after the charge is transferred is about -520 V since Vo.sub.1 is -500 V and V.sub.1 is -600 V in the unexposed part of the layer 1 before the charge is transferred. This residual potential becomes -640 V when V.sub.1 is -1300 V in the exposed part thereof.
FIGS. 1d and 2b show how the residual potential changes with a change in the dose of light applied. When the photosensitive plate 4 on which a residual potential of -540 V is present in the unexposed part is negatively charged again as in the process illustrated in FIG. 1a with the aim to use such photosensitve plate repeatedly, the charge potential dependent upon the impedance of the photoconductive sensitive layer 2 is superposed on the residual potential. This results in -830 V beng negatively larger than -600 V which is the potential in the unexposed part charged from zero potential for the first time.
The potential curve in the second charging is indicated by the broken line in FIG. 2a wherein the potential Ve approaches the saturated potential of the charger 5 when the dose of light is large. Hence, the potential after the second negative charging is about -1300 V which is equal to the charged potential for the first time. As a result, the potential in the unexposed part becomes further negative. Because the potential at the front insulating layer 1 in the unexposed part is higher than the transferring initiation potential -500 V even in the first turn of negative charging, the charge transfer to the recording paper 8 occurs also in the unexposed part. This causes a certain amount of toner to be attracted by the unexposed part in the process of development wherein the recording paper is placed in a liquid developer containing dispersed toner. Consequently, low signal-to-noise rato (S/N) recording results. In the second process of negative charging, the S/N becomes lower due to the fact that the potential in the unexposed part is negatively larger than that produced in the first turn.
In the prior art recording systems, wherein negative charging occurs only once, the unexposed part also is negatively charged to cause the recording S/N to be lowered and, in addition, the presence of the transferring initiation potential used in the process of electrostatic recording serves to further lower the S/N. This has hampered the repeated use of the photosensitive plate 4. It is for this reason that repeated recording with a high S/N could have hardly been realized by one charging process on an electrophotographically sensitive plate which comprises a front insulating layer.
One prior art approach to this problem is the adoption of a charge erasing process using an AC corona discharge or a DC charging process at opposite polarity to that of the residual charge.
In the former process, however, the AC corona discharge becomes unbalanced with respect to positive and negative polarities resulting in a certain amount of residual negative charge. In the latter process, it is impossible to maintain constant the potential at the surface of the front insulating layer.